OAuth2 Require Proof Key for Code Exchange (PKCE) Explained

PKCE (Proof Key for Code Exchange) is a security mechanism used in OAuth 2.0 Authorization Code Flow to prevent authorization code interception attacks.

It ensures that the authorization code can only be exchanged by the client that originally requested it, without requiring a client secret. PKCE is particularly useful for mobile apps, desktop apps, or scripts that cannot safely store a client secret.

Why is PKCE Needed?

Security Issue with Standard Authorization Code Flow

• Without PKCE, attackers could intercept the authorization code and use it to obtain an access token.
• In traditional OAuth2 flows, the client secret is used to verify the client—but public clients (like mobile/desktop apps) cannot store secrets securely.

How PKCE Solves This

• Instead of a client secret, PKCE uses a dynamically generated "code challenge" and "code verifier" to prevent code interception.
• Even if an attacker steals the authorization code, they cannot exchange it for a token without the correct code verifier.

How PKCE Works (Step-by-Step)

PKCE adds two new parameters to the standard OAuth2 Authorization Code Flow:

• "code_verifier" → A randomly generated string (used later in token exchange).
• "code_challenge" → A hashed version of the "code_verifier" (sent in the authorization request).

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Using Chilkat OAuth2 for PKCE (Proof Key for Code Exchange)

Chilkat OAuth2 natively supports PKCE with two simple property settings. There is no need to manually generate the "code_verifier" or "code_challenge".

How to Enable PKCE in Chilkat OAuth2

To use PKCE in your OAuth2 flow with Chilkat, set the following properties:

Chilkat.OAuth2 oauth2 = new Chilkat.OAuth2();

// Enable PKCE
oauth2.CodeChallenge = true;

// Specify PKCE method (SHA-256)
oauth2.CodeChallengeMethod = "S256";

// Other OAuth2 parameters
oauth2.AuthorizationEndpoint = "https://authorization-server.com/oauth2/authorize";
oauth2.TokenEndpoint = "https://authorization-server.com/oauth2/token";
oauth2.RedirectUri = "http://localhost:8080/callback";
oauth2.ClientId = "YOUR_CLIENT_ID";
oauth2.Scope = "openid profile email";

// Start authentication
bool success = oauth2.StartAuth();
if (!success)
{
Console.WriteLine(oauth2.LastErrorText);
return;
}

// ...
// ...

What Happens Internally?

• Chilkat automatically generates the "code_verifier"

• Chilkat derives the "code_challenge" using SHA-256

• OAuth2 request includes PKCE parameters ("code_challenge" and "code_challenge_method")

Why Use PKCE?

• Prevents authorization code interception

• No need for a client secret in public clients

• Good idea for mobile, desktop, and browser-based apps